1. Field of the Invention
The present invention relates to liquid crystal image creating systems. More specifically, the present invention relates to correction of linewidth deviations in images created in a liquid crystal by laser light.
2. Summary of the Prior Art
The present invention is applicable to any system that involves the creation of an image using laser light. A primary application of the present invention involves the laser generation of images on liquid crystal cells.
Referring to FIG. 1, a system is shown for the creation of an image on a liquid crystal display. The concept of creating an image on a liquid crystal display was first implemented at Bell Labs and is now known in the art. Generally, the process begins with creating a uniform pattern across the surface of the liquid crystal cell (a process depending on desired polarity commonly referred to as "darkening" or "brightening"). Once the surface of the liquid crystal cell 16 is made uniform an image may be created thereon. In one embodiment the image is created in an imaging computer 14. The imaging computer outputs digital signals which are sent to the laser unit 12. This digital output controls the firing of a laser which draws an image on the cell.
When the laser unit 12 is enabled a pulse of laser light is impinged upon the cell 16. The laser light creates an image by drawing a line on the cell 16. The direction of the line is controlled by a first and second galvanometer mirrors 18 and 20. Each galvanometer controls one axis. Therefore, by using two galvanometers a two dimensional image may be created. A more detailed description of the creation of an image on a liquid crystal cell is available in co-pending U.S. application Ser. No. 262,471 now abandoned.
Once the image is created on the cell 16, the image can be projected. In the formation of printed circuits a circuit board blank including a metal layer or film laminated on a suitable substrate is covered with a photosensitive layer. The cell 16 masks out light so that only certain portions of a photosensitive material or layer are exposed. The exposed photosensitive material is then removed leaving the exposed metal pattern. By etching, the exposed metal is removed, leaving the desired circuit.
When drawing a line on a liquid crystal cell using laser light the width of the line is crucial. For instance, in the printed circuit board context, if the line is too wide it may spill over into the next line and create a short circuit, or at a minimum create cross-talk. If the line is too narrow, there is a risk of a defect creating an open circuit, or having a line incapable of passing a requisite current.
The width of a line is controlled by the velocity at which the laser light passes over the cell 16 when creating the line. The slower the laser light is moving the wider the line is going to be. Conversely, the more rapidly the laser light is passed over the cell 16, the narrower the width of the generated line is going to be. Consistent with this relationship a general correlation exists between the width of the line created and the velocity at which the laser beam is swept across the surface of the cell 16. If it is desired to draw a line of 10 mils, a velocity of laser light correlative to 10 mils is selected. If it is desired to create a line of 5 mils the velocity correlative to a 5 mils wide line is selected.
Once an accurate correlation table is created for a particular image creation system, images having lines of varying width can be rapidly created. There are problems, however, in this arrangement with the creation of lines of uniform width. The four most prominent problems are (1) fluctuations in linewidth due to non-uniformities in the cell 16 itself, (2) variations in the angle of incidence of the laser light used to draw the line, (3) linewidth variation caused by the image projection system, and (4) elliptical shape of laser beam spot.
Addressing the problem of non-uniformity, over the surface of a liquid crystal cell small pockets of non-uniformity can exist. In these pockets the light gathering properties of the liquid crystal is slightly different from that of the surrounding liquid crystal. The non-uniform crystal may absorb more light than the surrounding crystal creating a wider line than desired. On the other hand, the non-uniform crystal may absorb less light than the surrounding crystal creating a narrower line than desired.
Significant variations are also caused by differences in angle of incidence of the laser light impinging on the cell. A laser light directly orthogonal to the surface of the cell 16 is going to have a different effect than light shown about the periphery of the cell 16 (at a larger angle of incidence). The greater the angle of incidence, the less intensity possessed by the laser light, but the greater the area of contact with the cell 16.
Additional linewidth variations can occur due to optical characteristics of the projection system. For example, astigmatism in the projection lens can cause differences between the widths of vertical and horizontal lines. Additionally, linewidth variation may be caused by lack of perfect circularity of the laser beam spot. For instance, if the spot is elliptical, the widest line results when the drawing motion is perpendicular to the long axis of the ellipse and the narrowest line results when the drawing motion is perpendicular to the short axis of the ellipse. These four sources of variations, and others, create differences in the linewidth as intended to be drawn and as actually drawn. As pointed out above, the effect of these variations can be quite significant.